Required Information
(32 Bytes)
Optional Information
(Not Required for USB)
Packet Information Word 0
Packet Information Word 1
Packet and Buffer Information Word 2
Buffer Information Word 0 (Buffer Length)
Buffer Information Word 1 (Buffer Pointer)
Linking Information (Next Descriptor Pointer)
Original Buffer Information Word 0 ( Buffer Length)Original
Original Software-Specific Information
( )2 Words(8 Bytes)
Original Buffer Information Word 1 ( Buffer )Original Pointer
Original Protocol-Specific Information
(0 to 64 Bytes in Multiples of 4 Bytes)
Original Private Data
(Any Number of Bytes in Multiples of 4 Bytes)
Functional Description
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• Length of the SOP data buffer
• Pointer to the next buffer descriptor in the
Packet descriptors can vary in size by design of their defined fields from 32 bytes up to 104 bytes. Within
this range, packet descriptors always contain 32 bytes of required information and may also contain 8
bytes of software specific tagging information and up to 64 bytes (indicated in 4 byte increments) of
protocol specific information. How much protocol specific information (and therefore the allocated size of
the descriptors) is application dependent. Port will make use of the first 32 bytes only.
From a general USB use perspective, a 32-byte descriptor size is suffix and the use of this size is
expected for a normal USB usage.
The packet descriptor layout is shown in Figure 16-13 and described within Table 16-9 through Table 16-
16.
Figure 16-13. Packet Descriptor Layout
Table 16-9. Packet Descriptor Word 0 (PD0) Bit Field Descriptions
Bits Field Name Description
The host packet descriptor type is 16 decimal (10h). The CPU initializes
31-27 Descriptor type
this field.
This field indicates the valid number of 32-bit words in the protocol-specific
26-22 Protocol-specific valid word count region. The CPU initializes this field. This is encoded in increments of 4
bytes as:
0 = 0 byte
1 = 4 bytes
...
16 = 64 bytes
17-31 = Reserved
The length of the packet in bytes. If the packet length is less than the sum
of the buffer lengths, then the packet data will be truncated. A packet
21-0 Packet length length greater than the sum of the buffers is an error. The valid range for
the packet length is 0 to (4M - 1) bytes. The CPU initializes this field for
transmitted packets; the DMA overwrites this field on packet reception.
1736
Universal Serial Bus (USB) SPRUH73H–October 2011–Revised April 2013
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